A related art non-volatile phase change memory device may utilize a phase change resistor with a variable resistance as a storage node. The resistance of the phase change resistor may vary depending on the crystalline state of the phase change resistor. For example, the resistance of the phase change resistor in a crystalline state may be lower, while the resistance of the phase change resistor in an amorphous state may be higher. A related art phase change memory device may use the phase change resistor to process 1-bit data. However, relatively higher heat treatment may be needed to change the phase change resistor from a crystalline state to an amorphous state, and thus, a relatively larger current or voltage need be applied to the phase change resistor.
The relatively higher operating current and/or voltage may limit the higher integration of phase change memory devices. For example, the higher operating current and/or voltage may induce a short channel effect for an access transistor controlling the phase change resistor, which may limit integration of the access transistor. In addition, integration of the phase change memory device may be limited by problems in manufacturing processes thereof, for example, etching damage in the phase change resistor and/or irregular formation of electrodes connected to the phase change resistor.
The related art discloses a multi-bit programming method for a phase change memory device in which the number of electric pulses applied to a phase change resistor is controlled. The resistance of the phase change resistor varies according to the number of applied electric pulses.
Other related art discloses a multi level cell (MLC) programming method in which the fall time of a current signal applied to a phase change resistor may be set to program the phase change resistor in one of a plurality of states. However, in related art programming methods, such as, the multi-bit programming method and the MLC programming method, the programming level may be increasingly difficult to controlled, and thus, related art programming methods may be less reliable and/or more difficult to commercialize.